I think we are kidding ourselves if we believe that the environment an autonomous car will encounter is anything like as simple as the environments that other software encounters at the moment. That other software has to deal with either very constrained environments, purely digital environments, or environments with few or no independent actors.
An autonomous car will have to work in a very complex (i.e. lots of automomous actors) analog (a multitude of, sometimes chaotic, behaviours) environment. Even other autonomous cars have analog characteristics as they are not purely software and the failures modes of their analog components, e.g. tyres, and their behaviours after a failure, are not easy to predict). And that is without considering the non-software-controlled actors (such as pedestrians, cyclists, even crisp packets** blown by the wind) that they will meet.
The "always on" mobile phone, by contrast, works with well-defined standards for its interfaces and the behaviours it's expected to encounter. And while the totality of software on a phone does have to deal with the user and many disparate tasks, each of those tasks is limited (limited, in many cases, by the design of the software***) and not (within the software) fully integrated with all of the others.
Aircraft fly in what are sparcely occupied and often almost-completely controlled environments (where, unfortunately, they sometimes do meet analog actors, such as flocks of birds).
As for ships, the so-called busiest shipping lanes (such as Strait of Dover) have nothing on the roads around me**** (and not only during the rush hour) For example, pedestrians' behaviour is very difficult to predict (and that's without considering those whose attention is focused solely on their mobile phone's screen).
Now it is not impossible for autonomous cars to work properly in the environment for which they're designed eventually, but we are kidding ourselves if we believe that this is going to happen safely soon. And there's no doubt that, in time, autonomous cars will be far less dangerous to their passengers, and other vehicles and humans in their vicinity, than we drivers are when we're behind the wheel. But the last thing we want to do is take the word of over-enthusiastic engineers (and those financing them and hoping for a return on their investment) that we should do anything as foolish as fast-tracking their mass introduction (as some are perhaps far too keen on doing).
** - Some radar-based safety systems have been known to cause a car to brake when a crisp packet has been blown in front of the sensor (though one would hope that either: a) future software will be able to cope with those flying packets; b) those packets are changed to something less environmentally unfriendly).
*** - If the software is not designed to recognise something, that something will not exist, as far as the software is concerned, in terms of the software's ability to deal with it other than as an exception.
**** - I live in a suburb of a small conurbation, and thus nowhere near the busiest place in the world.
